In a world facing ever increasing environmental and financial challenges there exist a growing need for more fuel efficient sea going vessels. Many of these vessels are multipurpose, performing a wide variety of tasks. A flexible vessel configuration is required to maintain optimal energy efficiency regardless of operating mode.
Conventional diesel mechanical and diesel electric propulsion systems both have their advantages and disadvantages. Combining the best of both propulsion systems and making a hybrid propulsion system is already a reality. Hybrid propulsion systems are becoming increasingly popular when choosing propulsion systems for new vessels for saving fuel and reducing emission of NOx, CO2 and other hazardous emission.
Known solutions of hybrid propulsion systems are, among others, US2008315583A which describes a propulsion system for a vessel having a diesel engine, a propeller, at least one auxiliary generator set, a generator and a drive unit for at least one machine, where the diesel engine can be connected on the one hand to the propeller and on the other hand by means of the generator to a power supply system in the vessel, where the drive unit for the machines have at least one converter which can be connected on the one hand to the power supply system and on the other hand to the load side of the generator.
Another known solution is described in WO09067722A, which describes a system and a method for providing power to a marine vessel and, more particularly, to a tugboat. The system includes diesel engines and generators and batteries which can be charged using power supplied by the generators, shore power or regenerated power. The tugboat is operated utilizing battery power only and the generators are used to provide additional power if needed or to recharge the batteries.
From WO08113999A it is known a propulsion arrangement for a marine vessel. The propulsion arrangement comprises an engine for propelling the vessel and an electrical machine coupled to the engine. The electrical machine is arranged to supply onboard electrical power for the vessel. A control unit controls the electrical machine such that the electrical machine is selectively operable as a generator or a motor. The control unit and the electrical machine are arranged such that the electrical machine, when operating as a motor, can supplement the power of the engine while the engine is in operation. In one embodiment, the control unit and the electrical machine are arranged to provide active damping of the engine torque.
In WO08130968A is described a marine vessel power generation and propulsion system including a control system. The system includes a plurality of generator sets, each generator set including an engine configured to drive an electrical generator and wherein each generator set is configured to supply electrical power to an electrical bus. The system includes further a control system configured to switch the power generation system between a plurality of operating modes, wherein in each mode of operation the control system adjusts each generator set to dynamically optimize the performance of the power generation system. In each mode of operation the control system is configured to prioritize a different predetermined characteristic when optimizing the performance of the power generation system.
A hybrid propulsion configuration allows both electrical motors and diesel engines to be used independently or combined to obtain optimum efficiency regardless of operating mode.
The main challenge, when using a hybrid propulsion system, is controlling the power flow and load sharing between diesel engines, electric motors and generators. This is particularly challenging when the propeller load is fluctuating and all the propulsion system components are linked through various gearboxes. A frequency converter is one option for controlling the speed and power output of electrical motors operating in parallel with a diesel engine through the same gearbox.
A serious limitation of the hybrid propulsion system is the common use of shaft generators to produce network electrical power. A shaft generator is driven by the same diesel engine powering the main propeller via a common gearbox. The problem occurs when the load on the main propeller is fluctuating whilst the diesel engine tries to maintain a constant speed to allow proper load sharing and load flow with other generators. Also a constant diesel engine speed is necessary for the shaft generator to maintain a stable network frequency. The result is energy inefficiency when propeller in many cases could be running with slower speed.
Other disadvantages with existing systems are that the gearbox needs two outputs; one for the electrical motor and one for the generator.
Further, a change of mode between operation of electrical motor and generator operation will be complicated and take long time.
A problem to be addressed is also to maintain selectivity at short-circuit of switchboard, something one today is dependent of a rotating machine, such as a generator, to achieve.